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my 05 x3 makes a metallic vibrating noise for 1 or 2 seconds at lows speeds when I depress the accelerator gently. Typically the engine revs are between 1500 and 2000 rpm.

The noise sounds like it is coming from underneath the car towards the back (maybe more towards the passenger side) and sounds like a faint metallic vibrating (like perhaps a loose heat shield).

Anybody get any ideas? I'm hoping it has something more to do with the exhaust system rather than the driveline. I'm hoping it gets a little worse before I bring it in - I don't want to get a "could not duplicate" response

I agree that a heat shield that has become loose is the most likely.
A muffler shop may be happy to look underneath for free and try and find it.

Also, lift up the cover of the load area where the battery is and see if anything there could be responsible. The jack or some other tool could be the cause.

If it comes down to the driveline, the best approach might be to first have an independent shop drain and flush the diff. and transfer case fluid. The state of the drained fluid might provide the diagnosis in the least costly way.

Thanks for the responses. I did read about the butterfly valve (that's at the very end of the muffler exhaust pipe, right?) but it seemed that was more of a cold weather/while idling noise.

It's hard to say, but the noise seems more on the passenger side, while the muffler is on the drivers. I'll bring it soon but I hope it gets worse so the tech can hear it. Sometimes I can make the noise happens, other times it doesn't do it. Unfortunately the window of opportunity to get this fixed under warranty is coming to an end shortly.

Unfortunately the window of opportunity to get this fixed under warranty is coming to an end shortly.

Not really. Keep in mind that dealers are allowed to self-authorize a goodwill approach up to 18 mos./75K miles after the warranty has expired.
(The BMW CACAP, S.I.B. 01 07 03)

It doesn't hurt to get them aware of what you hear before the warranty expires, but it is not mandatory.

Diagnostic work without an end-product is seldom profitable, or popular.
Not surprisingly, it's always better for a dealer's bottom line to be able to quickly replace an obviously failed component, regardless whether the customer is paying or BMWNA.

...How does the replacement muffler hold up? I'm a little bit concerned about where they weld it back to the exhaust. Isn't that usually the next weak point ?

I would say it is a random problem. I can remember my dad had the same noise on his 328. The butterfly valve would rattle on cold starts and smooth out as it warmed up. My 328 was exact same model and did not have the problem.

I was going to suggest that it could also be a loose bolt/nut that might rattle at specific rpm but that would remain even at operating temps. Basham and X3-terrestrial summed it up on the muffler - and you confirmed it.

Not a serious problem but most irritating I agree. Sure BMW will sort this out for you.

check and make sure your rubber hose/ vacume that connects to the butterfly is not leaking. I cut mine back about half inch today because the hose had a small cut right were it connected, it was sucking air and there for the butterfly was oscillating. it took care of the problem. nice and quiet

My 2004 X-3 is doing the same rattle. I have the extended 100k warranty so I think I'm gonna bring it in to have it replaced. Does BMW replace the entire muffler or just the vacuum line? I'm guessing it depends on what they find..